Pregnancy is arguably one of the most interesting examples of immune accommodation seen in mammalian biology. The placenta provides a barrier between the mother and child, regulating the reaction of maternal immune cells towards the allogeneic child at a time when its own immune system is immature or absent. Extravillous trophoblast (EVT) cells of the placenta invade deep into the maternal uterine tissue to establish a life giving connection with the maternal blood supply. It has been hypothesized that the interaction between EVT cells and maternal immune cells provides a controlling influence for implantation and development. We have been studying the biochemistry and immunology of the HLA-E, F, G class I molecules with a special focus on how they act individually and interactively, to help accommodate the maternal immune response to her developing offspring. We have recently shown that all placental cells that express HLA-G also express HLA-E and a subset of these also express HLA-F. Indeed, EVT cells that have invaded the maternal decidua are thus far the only normal cells known to simultaneously express all three nonclassical antigens. We propose to focus on features of HLA-E, F, and G that relate ultimately to their functions in the maternal-placental immune environment. Our current hypotheses concerning HLA-E, F, G in modifying a maternal immune response are being addressed in the following specific aims: 1) To examine the function of HLA-F in the peripheral immune response where we will decipher peptide or other ligand binding properties of HLA-F and identify a specific receptor interacting with HLA-F. This work is ultimately aimed at deciphering a specific role for HLA-F in the placental environment. 2) To test the ability of HLA-E bound to different peptide ligands, including the HLA-G nonamer, to evoke a specific immune response through binding an activatory receptor on maternal immune cells. This work will test the hypothesis that the HLA-G nonamer peptide bound to HLA-E confers a unique function on the complex necessary and specific for the placental immune environment. 3) To study soluble and membrane HLA-G with a major focus on determining their structural characteristics and interactions with inhibitory receptors. This work is aimed at testing the hypothesis that the primary function of HLA-G is to provide nonamer peptide to HLA-E and to otherwise act directly as a surrogate to classical class I interacting with inhibitory receptors expressed on decidual immune cells. We also propose to develop materials and reagents useful towards the further understanding of the structure, biochemistry, and function of HLA- E, F, and G.